docs: def available_within(self, start=None, end=None):
          """
          :return: Normal events that start within the range, and
           drop-in events that overlap the range.
          """
          return self.filter(event__is_drop_in=False).starts_within(start, end) | \
                 self.filter(event__is_drop_in=True).overlapping(start, end)